1. Field of the Invention
The present invention relates to fiber Bragg gratings and more particularly to a method of writing fiber Bragg gratings in infrared transmitting chalcogenide-based or chalcohalide-based fibers.
2. Description of Related Art
Since the discovery and description of photodarkening in chalcogenide glasses in 1971 by Berkes et al. ("Photodecomposition of Amorphous As.sub.2 Se.sub.3 and As.sub.2 S.sub.3 ", J. Appl. Phys. Vol. 42, No. 12, pp. 4908-4916, November 1971), much effort has been put forth to understand the detailed mechanisms of this effect. Sulfide based chalcogenide glass, specifically arsenic sulfide (As.sub.2 S.sub.3), exhibits a wealth of interesting permanent and reversible photoinduced changes when illuminated with light that has an energy near the Tauc gap of 2.3 eV. These changes include photodarkening, and photoinduced birefringence and dichroism. Photodarkening is discussed in "Mechanisms of Photodarkening in Amorphous Chalcogenides", K. Tanaka, Journal of Non-Crystalline Solids, Vol. 59-60, Part II, pp 925-928, (1983). Photoinduced birefringence and dichroism are discussed at greater length in "Photoinduced Optical Anisotropy in Chalcogenide Vitreous Semiconducting Films", V. G. Zhdanov et al., Physica Status Solidi (a), Vol.52, No. 1, pp 621-626, (March 1979) and in "Anisotropy of Photoinduced Light-Scattering in Glassy As.sub.2 S.sub.3 ", V. Lyubin et al., Journal of Non-Crystalline Solids, Vols. 164-166, pp 1165-1168, North-Holland (1993).
Although a unified theoretical microscopic description of these effects is not complete, it is believed that photodarkening is produced as carriers break As--S bonds when they recombine, causing an increase in As--As and S--S bonding, which in turn causes a lowering of the band-gap energy by as much as 0.05 eV at room temperature. (See "The Origin of Photo-Induced Optical Anisotrophies in Chalcogenide Glasses", H. Fritzsche, Journal of Non-Crystalline Solids, Vols. 164-166, pp. 1169-1172, North-Holland, 1993.) Since only a finite number of As--S bonds have a local environment which allows this process to happen, the effect saturates with total illumination energy. Regardless of the model, however, these effects are experimentally well characterized: the total refractive index change at 600 nm is about 0.01. (See "Photodarkening Profiles and Kinetics in Chalcogenide Glasses", S. Ducharme et al., Physical Review B, Vol. 41, No.17, pp. 12 250-12 259, Jun. 15, 1990.) A simple Kramers-Kronig analysis predicts that this index change will decrease linearly with photon energy in the transparent region of the glass, thus allowing large amplitudes (.DELTA.n.about.10.sup.-3) to be induced in the infrared.
The technique of side writing fiber Bragg gratings in germanium-doped silica fibers is well established and was first described by Meltz, et al. ("Formation of Bragg Gratings in Optical Fibers By A Transverse Holographic Method", Optics Letters, Vol. 14, No. 15, pp 823-825, August 1989.) Two "writing" beams are crossed at some angle .theta., with the intersection point coinciding with the core of the silica fiber. The crossed beams form an intensity grating along the axis of the fiber with period .LAMBDA.=.lambda..sub.W /(2 sin .theta.), where .lambda..sub.W is the wavelength and .theta. is the half-angle between the writing beams, respectively. The writing beams change an absorption line due to the germanium doping of the core, causing a change .DELTA.n in the index of refraction n at lower photon energies. The index change amplitude is around .DELTA.n.about.10.sup.-5 -10.sup.-6 for silica glass. This grating forms a Bragg reflector at the vacuum wavelength .lambda..sub.B for light launched down the core of the fiber at .lambda..sub.B =2n.LAMBDA.. The "photonic band gap" energy, .DELTA.v.sub.B, which corresponds to the full-width of the reflectance between the first two zeros of the reflectivity, is .DELTA.v.sub.B /v.sub.B =.DELTA.n/n where V.sub.B =c/.lambda..sub.B and c is the speed of light. See "Propagation Through Nonuniform Grating Structures", J. E. Sipe et al., J. Opt. Soc. Am. A, Vol. 11, No. 4, pp. 1307-1320 (April 1994).
It has been previously demonstrated by Shiramine et al. ("Photoinduced Bragg Reflector In As.sub.2 S.sub.3 Glass", Appl. Phys. Lett., Vol. 64 (14), pp 1771-1773, Apr. 4, 1994) that Hill gratings may be written in As.sub.2 S.sub.3 glass flakes. Hill gratings are formed from absorption of the peaks of the standing wave produced by multiple reflections from parallel end-surfaces. (See "Photosensitivity In Optical Fiber Waveguides: Application To Reflection Filter Fabrication", K. C. Hill et al., Appl. Phys. Lett., Vol. 32 (10), pp. 647-649, May 15, 1978.) The period of the standing wave sets the Bragg reflection condition, which gives a reflection maximum at the wavelength of the writing beam. Since the energy of the writing beam needs to be near the Tauc gap in order to photoinduce carriers, Hill gratings will not be useful at infrared wavelengths which are not significantly absorbed in the material.